feat(selection): M6.3 选股结果落库 + /api/selections（快照 + 逐候选行）

- domain/repositories/selection.py + SelectionMeta：选股 Repository Protocol
- models/selection.py + migration a6c91d4e7f20：selection_snapshot（查询/统计快照）+
  selection_result（逐候选：rank/score/factor_values/filter_status/reason JSON）
- repositories/selection_impl.py：save/get/list_recent（读回重建 SelectionResult）
- api/selections.py：POST /api/selections（同步执行+落库，返回 selection_id+result）、
  GET /{id}、GET 列表（as_of/method 过滤）；deps 装配 SelectionService/Repo
- config._build_mysql_url：仅编码破坏 URL 结构的字符（修 Alembic configparser 遇 %21 崩）
- 迁移已在 MySQL 应用（alembic head a6c91d4e7f20）；tests/test_selections_api.py 4 例
  提交→读回一致/404/列表过滤/condition；全量 pytest 通过

backend/app/api/router.py

@@ -8,13 +8,14 @@ from __future__ import annotations
 
 from fastapi import APIRouter
 
-from app.api import agent, experiments, factors, health, jobs, research, stocks
+from app.api import agent, experiments, factors, health, jobs, research, selections, stocks
 
 api_router = APIRouter()
 api_router.include_router(health.router)
 api_router.include_router(stocks.router)
 api_router.include_router(factors.router)
 api_router.include_router(research.router)
+api_router.include_router(selections.router)
 api_router.include_router(jobs.router)
 api_router.include_router(experiments.router)
 api_router.include_router(agent.router)